1. Field of the Invention
This invention relates to an emulsion polymerization inhibitor and a suspension polymerization method using the inhibitor. Further, this invention relates to resinous particles obtained by the suspension polymerization and uses therefor. More particularly, this invention relates to a technique in the suspension polymerization for producing resinous particles with a narrow particle size distribution and without heavy metal contamination.
2. Prior Art
Heretofore, as methods for the production of resinous particles, those resorting to mechanical pulverization, to suspension polymerization, and to emulsion polymerization have been known to the art. Of these methods, that which resorts to mechanical pulverization necessitates a huge energy input to pulverize the particles and require many classification steps to enable the resinous particles to be produced with a uniform diameter. Since the minute particles which are obtained by this method are amorphous morphologically, they have room for further improvement in flowability and proofness against flocculation. Although the method which resorts to emulsion polymerization is capable of producing minute particles of uniform diameter, the produced minute particles have a diameter of about 0.1 .mu.m and, therefore, cannot be directly put to such applications as mentioned above. In contrast thereto, the method which resorts to suspension polymerization allows relatively easy production of resinous particles of a desired diameter because it comprises preparing suspended particles of a monomer by mechanical stirring and subjecting the suspended monomer particles to polymerization. It further enjoys such advantages as obviating the necessity for using a solvent and facilitating the reaction control.
It has been known, however, that the suspension polymerization entails secondary formation in the aqueous phase of minute particles due to emulsion polymerization. The secondary reaction lowers the yield of the main polymerization and degrades the stability of this polymerization. Further, since the minute particles formed as described above by emulsion polymerization adhere predominantly to the phase boundary of the particles produced by suspension polymerization and cannot easily be completely removed therefrom, they suffer impairment of the physical properties of the produced resinous particles. Particularly when the resinous particles to be obtained by suspension polymerization are required to have such a small diameter as to be in the approximate range of 0.1 to 500 .mu.m, for example, and since the amount of a dispersion stabilizer to be added to the aqueous phase for ensuring stabilization of the minute suspended particles is large as compared with that for ordinary suspension polymerization, the amount of the polymerizable monomer dissolved in the aqueous phase at the step of dispersion and the step of polymerization is increased possibly to the extent of causing the problem of by-production of minute particles due to emulsion polymerization.
As a means for preventing the suspension polymerization from causing emulsion polymerization in the aqueous phase, the addition of an inorganic water-soluble inhibitor to the system has been known to the art. For example, JP-A-55-82,125(1980) discloses the addition of 0.01 to 10% by weight of a water-soluble inhibitor such as ammonium thiocyanate or cupric chloride to water, JP-A-60-8,302(1985) discloses the addition of vanadium pentoxide and/or cupric chloride in combination with a dispersion stabilizer, JP-A-62-205,108(1987) discloses the solution in water of not less than 10 ppm, based on the total amount of vinyl monomers, of such a water-soluble inhibitor as sodium nitrite, potassium nitrite, or cupric chloride, JP-A-2-284,905(1990) discloses suspension polymerization effected by the use of a water-soluble inhibitor such as a nitrite and a polymerization initiator formed of an organic peroxide, and JP-A-3-237,105(1991) discloses suspension polymerization effected in a continuous aqueous phase containing water, a water-miscible organic solvent, and a water-soluble polymerization inhibitor such as sodium nitride or hydroquinone.
Further, as disclosed in JP-A-61-255,353 (1986), the technique of adding to an aqueous suspension polymerization system a water-soluble mercaptan compound for the prevention of the sympathetic occurrence of emulsion polymerization has been known to the art. As water-soluble mercaptan compounds, 2-mercaptoethanol, thioglycolic acid, cysteine, glutathione, dimercaprol, 1,4-dithiothreitol, dimercaptosuccinic acid, and 2,3-dimercapto-1-propanesulfonic acid are cited in the specification in support of the disclosure.
JP-A-52-102,391(1977) discloses the addition of about 0.0005 to about 0.02 part by weight of a water-soluble inhibitor selected from among borohydrides represented by the following structural formula, alkali metal nitrites, alkaline earth metal nitrites, and ammonium nitrite and about 0.0001 to about 0.005 parts by weight of an oil-soluble inhibitor, oil-soluble and alcohol-soluble nigrosine, respectively based on 100 parts by weight of monomer. ##STR2## (wherein X is an alkali metal and R, R', and R" independently represent a hydrogen atom, a phenyl group, an alkoxy group, or an alkyl group of one to ten carbon atoms).
JP-A-5-61,253(1993) discloses the suspension polymerization under the presence of a water-soluble nigrosine. Further, JP-A-5-93005(1993) discloses the addition of a metal complex compound of monoazo dye represented by the following formula, in the suspension polymerization method which comprises preserving a continuous phase and a dispersive phase having at least a polymerizable monomer in separate containers, supplying at a controlled ratio these phases through respective passes to a size enlargement apparatus one or more times in order to obtain a suspension which contains polymerizable drops have a desired size, then introducing the suspension to a polymerizing container in order to finish the polymerization of the monomer, and the metal complex compound being added to the continuous phase. ##STR3## (wherein A, A' are independently a non-substituted or substituted phenylene group, B, B' are independently a non-substituted or substituted naphtyl group, M is a metal atom, R.sub.1, R.sub.2, R.sub.3, R.sub.4 are independently O, NH, or O--O.)
In the suspension polymerization of a polymerizable monomer such as a (meth)acryl type monomer, even when an inorganic water-soluble inhibitor is added to the reaction system, the effect of the inhibitor in preventing the emulsion polymerization is so weak that the inhibitor must be added in a large amount. Particularly, the conspicuity of this trend grows in proportion as the diameter of minute particles obtained by the suspension polymerization decreases.
When the water-soluble mercaptan compound mentioned above is used for preventing the sympathetic occurrence of emulsion polymerization, this mercaptan compound induces impartation of an offensive odor to the produced resinous particles or the effluent from the polymerization system. This offensive odor is not easily removed by washing.
As regards the use of the borohydride as a water-soluble inhibitor, this compound itself is difficult to handle and, on account of this difficulty, the conditions of the suspension polymerization dictate rigid control.
When the water-soluble nigrosine compound mentioned above is used, the effect of the compound in preventing the emulsion polymerization is also so weak that the compound must be added in a large amount. To add the compound in a large amount tends to shift the electrical charge property of the obtained particles to positive.
When the metal complex compound of monoazo dye mentioned above is used, some problems in the environmental pollution and the safety is grown up since the heavy metal such as chromium is contained in the polymerization system, and also in the resinous particles obtained.